linux-kernel - Re: [RFC][PATCH 0/2] Avoiding DWC3 transfer stalls/hangs when using adb over f

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Date:   Thu, 6 Feb 2020 10:41:34 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Felipe Balbi <balbi@...nel.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        John Stultz <john.stultz@...aro.org>,
        "Yang, Fei" <fei.yang@...el.com>,
        Andrzej Pietrasiewicz <andrzej.p@...labora.com>,
        lkml <linux-kernel@...r.kernel.org>,
        Felipe Balbi <felipe.balbi@...ux.intel.com>,
        Thinh Nguyen <thinhn@...opsys.com>,
        Tejas Joglekar <tejas.joglekar@...opsys.com>,
        Jack Pham <jackp@...eaurora.org>, Todd Kjos <tkjos@...gle.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Linux USB List <linux-usb@...r.kernel.org>,
        stable <stable@...r.kernel.org>, Jens Axboe <axboe@...nel.dk>
Subject: Re: [RFC][PATCH 0/2] Avoiding DWC3 transfer stalls/hangs when using
 adb over f_fs

On Thu, Feb 06, 2020 at 08:29:45PM +0200, Felipe Balbi wrote:
> > No, it shoudn't.  dma_map_sg returns the number of mapped segments,
> > and the callers need to remember that.
> 
> We _do_ remember that:

That helps :)

> that req->request.num_mapped_sgs is the returned value. So you're saying
> we should test for i == num_mapped_sgs, instead of using
> sg_is_last(). Is that it?

Yes.

> Fair enough. Just out of curiosity, then, when *should* we use
> sg_is_last()?

Outside of sg_next/sg_last it really shoud not be used at all as far
as I'm concerned.